Sculpted by: Arlen Pelletier
Release: 2019
Edition: Legacy
Used material(s): Cast in gold and silver-plated white metal
One of: Unlimited

Signed: Jay Fotos (first 300)

Reissued in Ghost Door box since 2020

First appearance: Head Games #2
Use: Allows the user to witness past events
Door: Timeshift clock

More information:
(possible spoilers)

The Timeshift Key was created sometime prior to 1912, by Harland Locke. Its original purpose is unclear, but Chamberlin Locke would use it as a way to revisit the history of his son, Ian. Chamberlin considered his use of the Timeshift Key to be a more responsible method to see his son, rather than the alternative, the Echo Key.
The Timeshift Key was used by the Tamers of the Tempest circa 1988 to gain information about the Black Door.
By the time the Locke children moved to Keyhouse, the Timeshift Key remained on top of the clock. When Tyler, influenced by Kinsey’s fears and tears, attempted to destroy Keyhouse, he knocked over the clock and discovered the key.
Tyler and Kinsey Locke first used the key to visit the earliest date they could, January 13, 1775. There, they learned the history of the Whispering Iron, Benjamin Locke, and the Black Door.
They then used the Timeshift Key to witness the events surrounding the Tamers of the Tempest in 1988, and the history of Dodge.

Uses:

The Timeshift Key allows the user to witness past events as chosen through the grandfather clock in Keyhouse. Though they cannot interfere with the past, it can be used to gain information.
The date can be selected using the clock itself, and the earliest date available is January 13, 1775, the date Benjamin and Miranda Locke first encountered the Black Door. The latest date available is December 31, 1999.
While time-traveling, the user can also leave the premises of Keyhouse. In the present, they appear “white” to witnesses, though this may be just to people who can see the magic.
Time in the present passes much slower than the time spent witnessing the past. When Tyler and Kinsey spent two days in the past, they only “went all white for a second” in the present.

Door:

The Timeshift Key’s door is the Timeshift Clock, a grandfather clock that is located on the stairway between the third and fourth floor of Keyhouse.

Guide to the Known Keys entry:

“…Occasionally people will ask me about Ian, wanting to know how I am managing without him. I tell them honestly that I see him everyday, that he is with me always, here in this house. This unusually earns me a consoling, worried look, but it is the perfect truth. With the aid of Harland’s ingenious Timeshift Key and Clock, I am able to revisit all of the best days of Ian’s life, as I wish. Of course I may also return Ian to our world with the Echo Key… but a human soul that has had its time on our earth longs for the warmth and comfort of That Other Place, and I know that bringing him back is something I would do for me, not for him. So I leave him be. I wonder, sometimes, what would happen if a truly twisted soul were to be reanimated with the Echo Key – someone with designs on the world of the living. Fortunately, I safeguard that key too closely for…”

Source: Locke & Key Wikia